Apparatus and method for atomizing liquid metal with viewing instrument

ABSTRACT

An apparatus and method for atomizing liquid metal are disclosed. A liquid metal supply is coupled to a nozzle for atomizing a stream of liquid metal in an atomizing zone extending from the nozzle. A viewing instrument provides a field of view extending to the atomization zone. A sensor coupled with the viewing instrument generates an image of the atomizing zone, and a control adjusts a flow rate of the stream responsive to the image.

This application is a division of application Ser. No. 08/037,848, filed03/29/93, now abandoned.

This application is related to applications Ser. No. 997,739, Ser. No.997,740, Ser. No. 997,742, Ser. No. 997,743, and Ser. No. 997,752 (nowU.S. Pat. No. 5,244,369), all filed 12/30/92.

This invention relates to an apparatus and method for atomizing liquidmetal.

BACKGROUND OF THE INVENTION

Close coupled gas atomization of liquid metal is being developed as aprocess for forming metal powders. The close coupled gas atomizationprocess is performed by a nozzle comprised of a melt guide tubeextending axially through a cylindrical gas plenum. The cylindrical gasplenum has an inner chamber in communication with an annular orifice, oran annular array of discrete orifices, so that a gas flow therethroughproduces an atomizing gas jet which may be comprised of an array ofdiscrete jets. The gas jet has a conical shape converging below the meltguide tube. A stream of liquid metal passing through the melt guide tubeand exiting therefrom is atomized by the conical gas jet converging inthe stream.

When the gas atomization of liquid metal is commenced, there is anopportunity to view the atomization of the liquid metal from viewportsin the atomization chamber. In the atomization process, the atomizinggas flows at supersonic speeds resulting in great scattering andrecirculation of the particulate formed by the atomization process. Soonafter the atomization starts producing powdered material, recirculatingpowder from the atomization process obscures the view. In fact, theobservation of the atomization nozzle is obscured within seconds afterthe process is started.

Information regarding the interaction between the atomizing gas and theliquid metal in an atomization zone below the nozzle can be obtained atthe start of the atomization process, and before the viewing path to theatomization zone is obscured by the recirculating powder produced by theatomization process. However, it has not been possible to view theatomization process for more than a few seconds after the atomizationhas begun. The ability to observe the interaction that occurs in theatomizing zone at and below the nozzle tip is lost. For example, oneproblem that can occur during the atomization process is liquid metalfreezing in the melt guide tube, herein referred to as freeze-off. Oftenthe freeze-off cannot be predicted or prevented resulting in undue delayand losses in the atomization process.

Several important properties of metal powder, and the products formedfrom consolidation of the powder, are dependent on the as-atomizedparticle size. These properties include composition homogeneity,mechanical performance, e.g. strength, and toughness, as well asphysical characteristics of the powder itself, e.g., particle shape,porosity, and flow qualities. Most of these properties improve asparticle size decreases, however, powder handling becomes morecomplicated for finer powder because of caking, environmentalcontamination, pyrophorosity and other affects.

The strong dependence of properties on particle size translates into anincreased demand for atomization process control that provides apredetermined particle size range, and minimizes the production ofpowder having a particle size above or below the predetermined range. Atthe same time, a number of variables necessarily change during theatomization process, such as the flow rate of molten metal through thenozzle as the static head pressure of the liquid metal in the cruciblechanges, temperature increase or decrease, and nozzle wear orconstriction. As a result, a series of adjustments can be requiredduring the atomization process in response to the changing variables. Wehave found that atomization process control can be improved by viewingthe interaction of the atomizing gas jet and the liquid metal stream inthe atomization zone. Such viewing can also be used to improve theprevention of freeze-off in the atomization nozzle.

U.S. Pat. No. 4,656,331, discloses an infrared sensor suitable for usein detecting the temperature of particles entrained in a plasma sprayjet. The sensor is used to control the electrical power input to theplasma torch to heat the particles to their melting temperature prior toimpact on a target substrate. U.S. Pat. No. 5,047,612, discloses anapparatus having an infrared sensor for controlling the deposition of apowder in a plasma spray process. A control means responsive to theinfrared sensor selectively adjusts a carrier gas flow rate in a powderinjection means for the plasma spray apparatus to selectively move thelocation of a powder impact point on a target. The infrared sensorgenerates an image representative of a temperature distribution of thepowder deposited on the target, the sensor having means for identifyinga location of an impact point of the powder upon the target.

An aspect of this invention is to provide an apparatus and method formonitoring and controlling the liquid metal atomization process.

Another aspect of the invention is to provide an apparatus and methodfor controlling the liquid metal atomization process by viewing theatomization zone, and adjusting as necessary one or more parameters,including the atomization gas pressure, to prevent freeze-off in themelt guide tube or provide a preselect powder size.

BRIEF DESCRIPTION OF THE INVENTION

The apparatus of this invention for atomizing liquid metal is comprisedof a liquid metal supply for providing a stream of liquid metal to anozzle coupled thereto for atomizing the stream in an atomizing zoneextending from the nozzle. A viewing instrument for providing a field ofview extending to the atomization zone. A sensor coupled with theviewing instrument for generating an image of the atomizing zone, and acontrol for adjusting a flow rate of the stream.

The method for atomizing liquid metal comprises, atomizing a stream ofmolten metal in an atomizing zone. Providing a field of view extendingto adjacent the atomizing zone. Generating an image of the atomizingzone from a position in the field of view, and selectively adjusting aflow rate of the stream responsive to the image.

As used herein, the term "image" or "imaged" means a visual display suchas a liquid crystal display or video monitor, or digitized data in aprocessor such as a computer.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description of the invention will be understood withgreater clarity if reference is made to the following drawings.

FIG. 1 is a schematic view of an apparatus for atomizing liquid metal.

FIG. 2-4 is a schematic view of a viewing instrument for viewing theatomization of liquid metal.

FIGS. 5 and 6 are each a schematic view of an apparatus for atomizingliquid metal having a control for the process.

FIG. 7 shows a temperature display of the atomizing zone along avertical cross section axial to the plume.

FIG. 8 is a graph showing the flow rate of water from the melt deliverytube as a function of atomizing gas flow rate in an atomizing nozzle.

DETAILED DESCRIPTION OF THE INVENTION

We have found that the atomized metal droplets formed by the closecoupled atomization nozzle form a finite plume. In addition, we havediscovered that a viewing instrument can be positioned adjacent theplume to provide a view of the atomization zone, without intruding intothe plume. As a result, a view of the atomization zone during theatomization process can be obtained despite the recirculating particlesin the atomization enclosure. Further, it has been found that images ofthe atomization zone obtained through the viewing instrument provideinformation characteristic of the atomization process. As a result,process control's can be adjusted in response to the atomization images,for example, to prevent freeze-off, or provide a preselect powder size.

Referring to FIG. 1, an apparatus 2 for atomizing liquid metal is shown.The apparatus 2 is comprised of a crucible 4, a nozzle 6, and anenclosure 8. The crucible 4 is formed of suitable material for holdingthe liquid metal, e.g. ceramic such as alumina or zirconia, or watercooled copper. A conventional heating means such as element 5 can bepositioned for heating the molten metal therein. The molten metal incrucible 4 can be heated by any suitable means, such as an inductioncoil, plasma arc melting torch, or a resistance heating coil. Thecrucible 4 has a bottom pouring orifice coupled with a melt guide tubein nozzle 6. The crucible 4, and nozzle 6 are conventionally mounted onatomization enclosure 8.

The atomization enclosure 8, formed from a suitable steel, is configuredto provide an inner chamber 9 suitable for containing the atomizationprocess. Depending upon the metal being atomized, enclosure 8 cancontain an inert atmosphere or vacuum. A suitable crucible enclosure 10can be formed over the crucible 4 to contain an inert atmosphere for theliquid metal. A conventional vacuum pump system, not shown, or gassupply, not shown, are coupled with atomization enclosure 8 and crucibleenclosure 10 to provide the inert atmosphere or vacuum therein. Aconventional exhaust system, not shown, for example with cycloneseparators, is coupled with enclosure 8 at connection 11 to remove theatomized powder during the atomization process.

A stream of liquid metal from crucible 4 is atomized by the nozzle 6,forming a plume of molten metal droplets 12 which are rapidly quenchedto form solid particulates of the metal. Suitable nozzles are shown, forexample, in U.S. Pat. Nos. 4,801,412, 4,780,130, 4,778,516, 4,631,013,and 4,619,845, incorporated herein by reference. The nozzle 6 directs astream of liquid metal into a supersonic jet of atomizing gas having aconical shape that converges in the melt stream. The high kinetic energyof the supersonic atomizing gas breaks up the stream of liquid metalinto atomized droplets which are widely dispersed in the atomizationenclosure. As a result, within several seconds of the initiation ofatomization, the atomization vessel is filled with a cloud ofrecirculating powder particulates, for example shown by arrows 14. Whileatomization of the liquid metal stream can be viewed at the initiationof atomization, for example from view port 16 mounted on atomizationenclosure 8, the interaction between the atomizing gas jet and theliquid metal stream is obscured by the cloud of metal particulateswithin a few seconds.

The apparatus and method of this invention are shown by making referenceto FIGS. 2-6. FIG. 2 shows an atomizing apparatus having common elementsidentified with the same number as in FIG. 1. A viewing instrument 30for viewing the atomization process extends through the enclosure 8. Theviewing instrument 30 extends through the atomization enclosure 8 to aviewing orifice 46 adjacent the atomizing zone 12. The viewing orifice46 should not extend into the atomizing zone, i.e. the plume of atomizeddroplets and supersonic atomizing gas jet. For example, in the atomizingapparatus shown in FIG. 2 it was found that the viewing orifice 46 couldextend to a position at least about 15 degrees, and about 20 millimetersfrom the axis of the atomized plume. Preferably, the viewing orifice 46is positioned about 30 to 60 degrees, and about 20 to 50 millimetersfrom the axis of the atomized plume.

Briefly described, one suitable nozzle 6 is comprised of a cylindricalplenum 32 and a melt guide tube 34 extending axially therethrough to amelt exit orifice 34a. A stream of liquid metal is poured through themelt guide tube 34 extending from the bottom pouring orifice in thecrucible 4. The plenum 32 defines an inner chamber 35 coupled with anannular atomizing gas orifice 35a spaced from the exit orifice 34a andconfigured to provide a jet of atomizing gas having a conical shapeconverging in the stream in the atomizing zone 12 extending from theexit orifice 34a. A gas inlet, not shown, extends through the plenum incommunication with the inner chamber 35. The gas inlet is coupled with aconventional gas supply, not shown, to provide the atomizing gas, suchas argon, to the inner chamber 35.

The atomizing zone 12 includes the exit orifice 34a at the tip of themelt guide tube 34 and extends therefrom, for example, for a distance ofup to about 20 exit orifice diameters. The atomizing zone typicallyincludes a first section 12a where the stream of liquid metal extendsfrom the exit orifice 34a. In a second section 12b, the liquid metalstream interacts with the gas jet emitted from the atomization gasorifice, and the stream breaks up or atomizes. The atomizing zone ischaracterized by a high kinetic energy from the atomization gas jet,which can be at supersonic speeds, and the plume of atomized liquidmetal droplets.

The viewing instrument 30 is configured to extend through the chamber toprovide a protected field of view extending to the atomizing zone 12.When a vacuum or protective atmosphere is maintained in the enclosure 8,the viewing instrument is formed with a window to provide a hermeticseal therein having a transparent window for the field of view to extendthrough. However, when atomization of metal is performed that does notrequire a protective atmosphere or vacuum, the viewing instrument can beformed so that atmosphere within the chamber 9 is in communication withthe ambient outside of the enclosure 8.

For example, the viewing instrument 30 can be comprised of a cylindricalsleeve extending through the enclosure to the viewing orifice adjacentthe atomizing zone. To maintain a vacuum in the enclosure 8, the viewinginstrument 30 can be comprised of a cylindrical first sleeve 38, atransparent window 42, and a second sleeve 44. The sleeve 38 extendsthrough the enclosure 8 to a first end 40 within the chamber 33. Thewindow 42 is mounted with conventional fasteners and a gasket tohermetically seal the first end 40. The window 42 is formed from atransparent material such as silica glass, Lucalox polycrystallinealumina, polycrystalline yttria for example shown in U.S. Pat. No.4,755,492, and single crystal alumina or zirconia.

The second sleeve 44 is mounted on the first end 40, and extendstherefrom to the viewing orifice 46 spaced from the atomizing zone 12.Preferably, the second sleeve 44 is formed to have a conical shapenarrowing at the viewing orifice 46. The second sleeve 44 can be formedfrom a suitable metal sheet, such as steel or aluminum, that is rolledand spot welded to form the cylindrical or conical shape. Preferably,the second sleeve 44 extends at least about 30 centimeters from thewindow 42 so that recirculating particles entering the viewing orificefall to the bottom of the second sleeve before hitting the window. Thesecond sleeve 44 is mounted on first end 40, for example by extendingthe second sleeve around first sleeve 38 and securing thereto with aconventional fastener such as a clamp.

The viewing orifice 46 is of a suitable size, for example about 10 to 40millimeters in diameter, to provide a view of the atomizing zone 12 fromwithin the first sleeve 38. A sensor 50 for generating an image of theatomization zone, such as a CID or CCD video camera, can be mounted onthe enclosure 8, or within the first sleeve 38, so that the field ofview of the sensor 50 extends through the window 42. The sleeve 38 andsecond sleeve 44 being positioned to extend through enclosure 8 so thatthe field of view of sensor 50 extends through the atomizing zone 12.

The first sleeve can be formed with a first section extending from theenclosure, and a second section extending to the first end 40. Thesecond section extending into the first section to provide for axialadjustment of the position of the first end 40. A conventional flangehaving a gasket is conventionally mounted, for example with matingthreads, to provide for axial adjustment and hermetic sealing betweenthe first and second sections.

Recirculating powder particles from the atomization process can enterthe second sleeve 44 through viewing orifice 46 and deposit on window 42or scratch the window obscuring the view from within the first sleeve.Preferably, a gas purge, not shown, is mounted in the second sleeve tosweep the particulate from the transparent window, and out of the secondsleeve. A suitable gas purge can be formed by extending a tube into thesecond sleeve 44. The tube is operatively coupled with a conventionalgas supply, and the tube is positioned in the second sleeve to direct aflow of inert gas across window 42 to sweep the particulate therefrom.For example, it was found an argon flow rate of 1.5 Kg/min. from thetube was suitable to keep the window clean during atomization of a steelalloy. Preferably, the second sleeve 44 is formed with a bore to allowthe purging gas and atomized particles to pass therefrom.

Other embodiments of the viewing instrument used in the method andapparatus of this invention are shown in U.S. patent applications Ser.No. 07/997,740, Ser. No. 07,997,739, and Ser. No. 07/997,752,incorporated herein by reference.

Briefly described, another viewing instrument is shown by referring toFIG. 3. The viewing instrument 60 is comprised of an annular housing 62extending from a first end 64 to a second end 66 defining an innerchannel 68. Conventional fasteners 70 extending through mating threadedbores in housing 62 secure a camera 72 therein to have a field of viewextending from the second end 66. A sleeve 74, extends from a third end76 to a fourth end 78 defining a second inner channel 80. The sleeve 74is mounted on the housing 62 so the field of view of the camera 72extends through second channel 80, and the third end 76 faces second end66. A transparent window 82 is positioned between the housing 62 and thesleeve 74, and mounted to seal the first channel 68 from the secondchannel 80. Conventional lensing for the camera 72 can be positioned ata front end 72a of the camera, or the transparent window 82 can beformed as the lensing for the camera.

The sleeve 74 is formed with a passage 84 extending through the sleevein communication with the window 82. A tube or tubes 87 are mounted toextend into the passage 84 and are operatively coupled to a conventionalgas supply not shown. The passage 84 is configured so that a gas flowpassing from the tubes 87 into the passage 84 directs a stream or jet ofgas from the window 82 towards the fourth end 78 of sleeve 74. The gasflow or jet through the second channel 80, protects the transparentwindow 82 from atomized particles depositing on, scratching, orotherwise damaging the window. The viewing instrument 60 can be mountedon a bracket in the atomizing enclosure so that the sleeve 74 is spacedfrom the atomizing zone, and the field of view from the camera withinthe housing 62 extends through the atomizing zone. A flexible sleeve 88extends from the first end 64 to the enclosure to protect the cameracord 90.

Another embodiment of a viewing instrument that can be used in themethod and apparatus of this invention is shown by making reference toFIG. 4. The nozzle 6 is comprised of the cylindrical plenum 32, and themelt guide tube 34 extending axially therethrough to the melt exitorifice 34a. The plenum 32 defines the inner chamber 35 coupled with theannular atomizing gas orifice 35a spaced from the exit orifice 34a andconfigured to provide an annular jet of atomizing gas converging in theatomizing zone 12 extending from the exit orifice 34a. The plenum 32 isconfigured with a cylindrical inner sidewall 100 extending below theexit orifice 34a and spaced from the atomizing zone 12. The innersidewall 100 extends to a cylindrical outer sidewall 102. A gas inlettube 104 extends through the outer sidewall 102, and is coupled with aconventional gas supply, not shown, to provide an atomizing gas to theinner chamber 35.

The viewing instrument comprises the nozzle 6 having a cylindricalsleeve 106 extending through the outer sidewall 102 to a first end 108in communication with the inner chamber 35. A camera 110 is mounted byconventional fasteners 112 in the sleeve to have a field of view 120extending from the first end 108. The sleeve 106 is positioned so thatthe field of view 120 of the camera 110 can extend through the atomizingzone 12. A transparent window 114 is mounted to seal the first end 108of the sleeve 106. A porthole 118 is formed in the inner sidewall 100along the field of view 120 of camera 110 to provide a view of theatomizing zone through the inner wall 100.

For example, the porthole can be formed as a bore in the inner sidewall100 along the field of view of camera 110 to provide a view of theatomizing zone 12 to camera 110. Atomizing gas at a high pressure in thechamber 35 flows from the porthole 118, and interacts with recirculatingpowder particles that can be directed down the porthole. The flow or jetof gas emitted from the porthole blows the powder particles back out ofthe porthole before the powder can reach the transparent window 114. Theflow or jet of gas from the porthole prevents powder from contaminating,e.g., pitting, scratching, or depositing on the transparent window 114.

The image of the atomizing zone can be any image that can be formed fromthe reception of electromagnetic radiation emanating or reflected fromthe atomizing zone. For example, the infrared wavelengths from theliquid metal can provide temperature information, and the visiblewavelengths from luminescent liquid metals can provide visualinformation that can be used to image the atomization zone. The sensorcan be any conventional apparatus for generating the image, such as avideo camera, pyrometer, or imaging radiometer, for example shown inU.S. Pat. Nos. 4,687,344, or 4,656,331, incorporated herein byreference. The imaging radiometer can provide a temperature profile, ormap of the atomizing zone with an associated visual image.

Preferred commercially available video cameras are miniature CCD basedS-VHS or VHS cameras having high speed electronic shutters. Examples ofcommercially available video cameras are the Toshiba IKM-30A, M-40A, andIKM-M30MA, and the Sony XC-77RRCE and SC-RRCE. Such cameras can have adiameter of 3 centimeters or less, and a length of about 6 centimetersor less complete with the associated lensing. As a result, the cameracan be positioned in a small space. Conventional lensing for the cameracan be positioned at a front end of the camera, or the transparentwindow in the viewing instrument, e.g. discussed above, can be formed asthe lensing for the camera. An example of suitable conventional lensesfor the miniature cameras are microminiature lenses, such as, Toshibamodels JK-L04, JK-L7.5, JK-L15, and JK-L24.

The sensor provides an output signal that can be used to generate theimage of the atomization zone. The output signal can be sent to adisplay, or sent to a processor such as a computer to analyze andcompare the image to a predetermined reference pattern or patterns. Inone embodiment the sensor is a pyrometer which outputs a voltage signalcorresponding to a temperature or series of temperatures in theatomizing zone. The voltage signal can be sent to a display such as aliquid crystal to display the temperature or series of temperatures toprovide a thermal profile of the atomizing zone. In addition, thevoltage signal can be sent to a processor such as a computer where it isdigitized and compared to a predetermined reference value or values todetermine if there is a deviation larger than a given amount. If such adeviation exists the computer is used to send control signals to adjustthe flow rate of the liquid metal stream in order to bring the sensedtemperature back within the limits of the predetermined referencevalues. It should be understood that the voltage signal can be sent tothe display or the processor, or both.

Referring to FIG. 5, in a more preferred embodiment the sensor 50 is avideo camera which can provide the standard EIA RS-170 composite signal(525 line, 60 Hz, 2/1 interlace), and the video signal can be sent to avideo monitor 202 to display an image of the atomizing zone 12. Thevideo signal can be processed further to display more information. Forexample, when the camera 50 is configured as an imaging radiometer, thevideo signal can be sent to a video signal processor 200 prior todisplay on the video monitor 202. In a preferred embodiment, the videosignal processor is an analog video analyzer such as the Colorado VideoModel 321 Video Analyzer, Boulder, Co., for example, providing acontinuous graphical display of signal intensity, i.e., atomizing zonetemperature, along a user-selected cursor as well as additional signaloutputs useful for further processing. The user positioned cursorintersects the image of the atomizing zone 12 along a section line, andthe temperature variations along the cursor line are displayed. Thevideo signal can also be sent to a video recorder to provide a record ofthe atomizing process.

The output signal from the camera 50 can be sent to a computer, orpreferably as shown in FIG. 6, to both the video monitor 202 and thecomputer 210 for example by splitting the signal at the video signalprocessor 200. The video output signal sent to the computer 210 isconverted from analog to digital in the video signal processor 200, forexample by a conventional frame grabber card, for example a PC VisionCard and software marketed by Imaging Technology, Inc. for IBM PC-Classcomputers. Preferably, the video signal processor 200 is mounted in thecomputer 210.

The computer 210 is preferably configured to compare the informationfrom the video signal processor to a predetermined reference image orimages of the atomization zone to determine if there is a deviationlarger than a given amount. The computer 210 can be programmed withconventional pattern recognition software to recognize the predeterminedreference image or images from the atomizing zone that can be used tocontrol the atomization process. A suitable pattern recognition softwareis Image Analyst, available from Automatix, Billerica, Mass. If such adeviation exists the computer is used to send control signals to adjustthe flow rate of the liquid metal stream in order to bring the sensedimage back within the limits of the predetermined reference image orimages.

We have discovered that the apparatus of this invention provides imagesof the atomization process with a resolution that permits identificationof characteristics of the atomization process. For example, we havediscovered characteristic images that warn of impending freeze-off inthe melt guide tube as much as 30 seconds before the freeze-off occurs,allowing sufficient time to selectively adjust the flow rate of theliquid metal stream and prevent the freeze-off. Images of a buildup ofsolid metal at the exit orifice, or a divergence of the atomized plumeaxis from the axis extending from the melt guide tube both indicateimpending freeze-off in the melt guide tube. As a result, it has nowbeen found that freeze-off is not a sudden unpredictable event, and themethod and apparatus of this invention can be used to detect freeze-offwarning events to provide for control of the atomization process toprevent the freeze-off.

By using high speed video cameras having a shutter speed of about 0.002seconds or less to observe the atomization zone through the viewinginstrument, a number of additional characteristics of the atomizationprocess were discovered. For example, plumes having a greater spread ata given distance from the exit orifice produce a higher yield of fineparticulates, as compared to plumes having a narrower spread at the samedistance. In addition, it was found that the gas jet interacts with theliquid metal stream to form a webbing of substantially interconnectedligaments. The ligaments are then further fragmented to form theatomized droplets. Plumes comprised of a higher density of smallerligaments have an improved yield of fine particulates, as compared toplumes comprised of a lower density of larger ligaments.

When the camera is configured as an imaging radiometer, a thermalprofile of the atomizing zone can be imaged. Preferably, a near infraredfilter admitting radiation having a wavelength of about 0.8 to 1.1microns is employed in the imaging radiometer. The video signal outputby the camera is proportional to the radiant power flux incident uponthe detector in the camera. The video signal from the camera can bedisplayed on a television monitor resulting in a continuous gray scaledepiction of temperature variations of the target. FIG. 7 is a sketch ofthe imaging radiometer's display, as modified by the Video Analyzerdescribed above. The user positioned cursor 230 is positioned to providethe thermal profile along an axial cross section of the atomizing zone.FIG. 7 shows the signal intensity along the cursor line 230 increases toa sharp peak 232 at the exit orifice 34a of the melt guide tube 34during atomization. It has been found that when the signal peak 232 atthe exit orifice 34a decreases or disappears, it is a warning ofimpending freeze-off in the melt guide tube 34.

The atomization process can be controlled by selectively adjusting theflow rate of the liquid metal stream directed through the atomizingnozzle. For example, freeze-off in the melt guide tube can be averted byincreasing the flow rate of the liquid metal stream. The increased flowrate of the liquid metal stream helps to remove deposits that havesolidified in or on the melt guide tube. In addition, the average powdersize can be changed by changing the liquid metal flow rate for a givengas flow rate through the nozzle.

Referring to FIG. 5, the flow rate of the liquid metal stream can beadjusted by changing the atomizing gas flow rate, or pressure in theplenum 32. Various nozzle configurations having different gas jets, orpositioning of the gas jets with respect to the exit orifice on the meltguide tube, provide a unique liquid metal flow rate response as theatomizing gas flow rate changes. For example, FIG. 8 shows the flow rateof water through a nozzle such as shown in FIG. 5, as the atomizing gasflow rate increases. Although FIG. 8 shows the flow rate of waterthrough the nozzle, the flow rate of a liquid metal through the nozzlewill follow a similar trend as the atomizing gas flow rate changes.

FIG. 8 shows the maximum liquid flow rate at an atomizing gas flow rateof about 9 to 11 Kg/min. Therefore, when the atomizing gas pressure isabove the peak, e.g., 14 kg/min., the atomizing gas pressure is loweredto increase the liquid flow rate through the melt guide tube. When theatomizing gas pressure is below the peak, e.g., 7 kg/min., the atomizinggas pressure is increased to increase the liquid flow rate through themelt guide tube. A first gas supply 204 operatively coupled to a gasinlet in the plenum 32 can be selectively adjusted to increase ordecrease the atomizing gas pressure in the plenum in response to theimage, e.g. on monitor 202.

In addition, the flow rate of the liquid metal stream can be adjusted bychanging the ambient pressure within the crucible enclosure 10. A secondgas supply 206 is comprised of a conventional gas supply coupled to agas inlet in the crucible enclosure 10. The second gas supply 206 isselectively adjusted to increase or decrease the gas pressure in thecrucible enclosure 10. For example, the increased gas pressure withinthe crucible enclosure 10 increases the hydrostatic pressure of theliquid metal within the crucible 4. As liquid metal is poured from thecrucible 4 and the depth of liquid metal in the crucible is reduced,reducing the hydrostatic pressure of the melt, the increased gaspressure within the crucible enclosure compensates for the reducedhydrostatic pressure of the melt to increase the flow rate of the liquidmetal stream through the melt guide tube.

A first control 212 and second control 214 are coupled to the first andsecond gas supplies, respectively, for selectively adjusting the gasflow supplied to the plenum 32 and crucible enclosure 10. For example,the controls 212 and 214 can be electric or pneumatic activated valves.The controls can be manually operated in response to the sensed image,for example displayed on video monitor 202. In a preferred embodimentshown in FIG. 6, the controls are coupled to the computer 210, and thecontrol signals from the computer selectively adjust either or both ofthe controls 212 and 214 to adjust the flow rate of the liquid metalstream in order to bring the sensed image back within the limits of thepredetermined reference image or images.

What is claimed is:
 1. A method for controlling the atomization zoneduring an atomization process comprising the steps of:storing at leastone image of at least one reference atomization zone process parameter;atomizing a stream of a liquid metal having a flow rate with anatomizing fluid in the atomization zone; viewing the atomization zonewith means capable of generating at least one image thereof; generatingat least one image of the atomization zone; comparing the at least onegenerated image to the at least one stored image of the at least oneprocess parameter; determining the amount of deviation of the generatedimage from the stored image; and when the generated at least one imagedeviates from the at least one stored image, selectively adjusting theatomization zone such that the atomization zone more closely resemblesthe stored image of the atomization zone.
 2. The method of claim 1wherein the generated image is a thermal plot of the atomization zone.3. The method of claim 1 wherein the generated image is a video image ofthe atomization zone having an exposure time of about 0.002 seconds orless.
 4. The method of claim 1 further comprising the step of:directingthe stream through a nozzle comprised of a cylindrical plenum and a meltguide tube extending axially therethrough to an exit orifice, the plenumbeing configured to provide the atomizing fluid to have a conical shapeconverging in the stream to form the atomization zone.
 5. The method ofclaim 1 further comprising the steps of:directing the stream through anozzle comprised of a cylindrical plenum, and a melt guide tubeextending axially therethrough to an exit orifice; coupling a cruciblehaving a pouring orifice with the melt guide tube; placing an enclosurefor containing an atmosphere over the crucible; and adjusting the flowrate of the stream by a second gas supply coupled with the crucibleenclosure to provide a selected atmosphere pressure therein.
 6. Themethod of claim 1 further comprising the steps of:directing the streamthrough a nozzle comprised of a cylindrical plenum and a melt guide tubeextending axially therethrough to an exit orifice, the plenum beingconfigured to provide the atomizing fluid to have a conical shapeconverging in the stream; providing a first gas supply coupled with theplenum such that an atomizing gas pressure is maintained therein;providing a crucible having a pouring orifice coupled with the meltguide tube; and providing an enclosure for containing an preselectatmosphere over the crucible coupled with a second gas supply forproducing an atmosphere pressure therein, such that the flow rate of thestream is adjusted by the first and second gas supplies.
 7. A method forcontrolling the atomization of liquid metal in a close coupled gasatomization apparatus, the method comprising the steps of:providing anenclosure having a chamber for containing particulates formed fromatomized liquid metal; providing a nozzle, mounted on the enclosure incommunication with the chamber, for atomizing the liquid metal, thenozzle including a plenum means and a melt guide tube extending axiallytherethrough to an exit orifice, the plenum means allowing atomizing gasto converge in an atomization zone extending from the exit orifice;positioning viewing means in the enclosure for providing a view of theatomization process including the tip of the melt guide tube;positioning at least one process sensor in the viewing means; storingimages of a plurality of reference atomization process parametersincluding at least one which indicates impending freeze-off; monitoringthe atomization process with the process sensor; generating images ofthe atomization process; comparing the generated atomization processimages to the reference atomization process parameters images;determining the amount of deviation of the generated atomization processimages from the reference atomization process parameters images; andwhen the generated images of the atomization process deviates from thestored reference atomization process parameter images, changing at leastone atomization process parameter thereby resulting in a change of thegenerated images of the atomization process to coincide with the storedreference atomization process parameter images.
 8. The method of claim 7wherein the viewing means is positioned about 15 degrees to about 60degrees from the axis of the atomization zone.
 9. The method of claim 8wherein the viewing means is positioned about 30 to about 60 degreesfrom the axis of the atomization zone.
 10. The method of claim 7 whereinthe viewing means is positioned about 20 millimeters from the axis ofthe atomization zone.
 11. The method of claim 7 wherein the viewingmeans is positioned about 20 to about 50 millimeters from the axis ofthe atomization zone.
 12. The method of claim 7 wherein the processsensor is a video camera.
 13. The method of claim 7 wherein the processsensor is a pyrometer.
 14. The method of claim 7 wherein the processsensor is an imaging radiometer.
 15. The method of claim 7 wherein,after determination of the deviation from the at least stored referenceatomization process parameter which indicates impending freeze-off, butprior to actual freeze-off, changing at least one atomization processparameter sufficiently to prevent the impending freeze-off.
 16. Themethod of claim 7 wherein, during the image generating step, images of abuildup of solid metal proximate the tip of the melt guide tube aregenerated about 30 seconds prior to freeze-off occurring.
 17. The methodof claim 16 further comprising the step of:after generation of images ofthe metal buildup proximate the melt guide tube, adjusting the flow rateof the liquid melt stream so that freeze-off is averted.
 18. The methodof claim 7 further comprising the step of:comparing the generated imagesof an atomization plume spread at least one distance from the end of themelt guide tube with the stored images of atomization plume spreads atthe same distance from the end of the melt guide tube.
 19. The method ofclaim 18 further comprising the step of:adjusting the atomization plumespread at the one distance from the end of the melt guide tube such thatrelatively higher yields of fine particles are produced during theatomization process.
 20. The method of claim 7 further comprising thestep of:in response to the generated images, adjusting the atomizationapparatus such that a webbing of substantially interconnected ligamentsis produced.
 21. The method of claim 7 further comprising the step of:inresponse to the generated images, adjusting the atomization apparatus toproduce an atomization plume having a relatively higher density ofsmaller ligaments such that a higher yield of fine particles isproduced.
 22. The method of claim 7 further comprising the stepof:detecting plume temperature variations.
 23. The method of claim 22wherein during the plume temperature variation detection step, thedetected temperature at the exit of the stream of liquid metal from themelt guide tube is the peak temperature detected.
 24. The method ofclaim 23 wherein when the detected peak temperature at the exit of thestream from the melt guide tube decreases from the peak temperaturedetected, freeze-off in the melt guide tube is impending.
 25. The methodof claim 7 further comprising the step of:in response to detecteddeviations in the generated images of the atomization process from thestored reference atomization process parameter images, increasing theflow rate of the liquid melt stream such that freeze-off is averted. 26.The method of claim 7 further comprising the step of:in response todetected deviations in the generated images of the atomization processfrom the stored reference atomization process parameter images,increasing the flow rate of the melt such that solid deposits in or onthe melt guide tube are at least sufficiently reduced, such thatfreeze-off is averted.
 27. A method for controlling the atomization ofliquid metal in a close coupled gas atomization apparatus, the methodcomprising the steps of:providing an enclosure having a chamber forcontaining particulates formed from atomized liquid metal; providing anozzle, mounted on the enclosure in communication with the chamber, foratomizing the liquid metal, the nozzle including a plenum means and amelt guide tube extending axially therethrough to an exit orifice, theplenum means allowing atomizing gas to converge in an atomization zoneextending from the exit orifice; positioning viewing means in theenclosure for providing a view of the atomization zone including the tipof the melt guide tube; positioning at least one process sensor in theviewing means; storing images of a plurality of reference processparameters; monitoring the atomization zone with the process sensor;generating images of the atomization zone including the tip of the meltguide tube; comparing the generated images of the atomization zone toimages of the reference process parameters; determining the amount ofdeviation of the generated images from the reference images; andadjusting the atomization apparatus such that a webbing of substantiallyinterconnected ligaments is produced.
 28. The method of claim 27 whereinin response to the generated images, adjusting the atomization apparatusto produce an atomization plume having a relatively higher density ofsmaller ligaments such that a higher yield of fine particles isproduced.